Understanding language is one of the most fundamental human cognitive abilities. It plays an important role in normal development, and is a major means for acquiring information in many domains. Psycholinguists have made significant progress in clarifying the structures and processes that underlie language comprehension. In recent years, it has become clear that a central issue in this area is lexical access. Theories most specify how the presentation of a spoken word leads to a particular lexical representation becoming activated, and what the effects of such activation are: What effect does one active lexical representation have on others, and on units at other levels of representation? The current proposal includes a large set of theoretically-driven empirical studies of lexical activation. The experiments use a range of different methodologies, in order to assure correct theoretical inferences through converging operations. Several of the experiments investigate structural issues: What is the nature of the connections between lexical entries? What types of input and output codes are involved in accessing lexical representations? Are the same unit types (e.g., phonemes, demisyllables, and syllables) used by both input (perception) and output (production) processes? Other experiments explore aspects of lexical processing: Is lexical access a purely bottom-up, autonomous process, or is the architecture interactive, with top- down information flow? Does processing proceed in a strictly "left-to-right" fashion, or does the system instead use some forms of asynchronous processing? More generally, what is the time course of lexical processing? The product of the proposed research will be a much better understanding of the architecture of the system that accomplishes language comprehension. Such an understanding is critical to our understanding language processing. In turn, because language is such a fundamental cognitive ability, progress in describing language processing will enhance our understanding of human cognition.